Super hard components and powder metallurgy methods of making the same

ABSTRACT

A method of forming a super hard polycrystalline construction comprises forming a liquid suspension of graphene and grains of super hard material, dispersing the graphene and super hard grains in the liquid suspension to form a substantially homogeneous suspension which is dried and from which a pre-sinter assembly is formed and then treated to create a sintered body of polycrystalline super hard material comprising a first fraction of super hard grains and a second fraction of diamond grains, the graphene being at least partially converted to diamond during the sintering stage to form the second fraction. The super hard grains in the first fraction are bonded along at least a portion of the peripheral surface to at least a portion of a plurality of diamond grains in the second fraction, and have a greater average grain size than that of the grains in the second fraction which is between 60 nm to 1 micron.

FIELD

This disclosure relates to super hard constructions and methods of making such constructions, particularly but not exclusively to constructions comprising polycrystalline diamond (PCD) structures which may or may not be attached to a substrate, and tools comprising the same, particularly but not exclusively for use in rock degradation or drilling, or for boring into the earth.

BACKGROUND

Polycrystalline super hard materials, such as polycrystalline diamond (PCD) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of super hard tool inserts may be limited by fracture of the super hard material, including by spalling and chipping, or by wear of the tool insert.

Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and a super hard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process. The substrate is generally formed of a tungsten carbide-cobalt alloy, sometimes referred to as cemented tungsten carbide and the super hard material layer is typically polycrystalline diamond (PCD), or a thermally stable product TSP material such as thermally stable polycrystalline diamond.

Polycrystalline diamond (PCD) is an example of a super hard material (also called a super abrasive material or ultra hard material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1,200° C., for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent—catalysts for PCD sintering.

Cemented tungsten carbide, which may be used to form a suitable substrate, is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify. To form the cutting element with a super hard material layer such as PCD, diamond particles or grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains occurs, forming a polycrystalline super hard diamond layer.

In some instances, the substrate may be fully cured prior to attachment to the super hard material layer whereas in other cases, the substrate may be green, that is, not fully cured. In the latter case, the substrate may fully cure during the HTHP sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the super hard material layer.

Ever increasing drives for improved productivity in the earth boring field place ever increasing demands on the materials used for cutting rock. Specifically, PCD materials with improved abrasion and impact resistance are required to achieve faster cut rates and longer tool life.

Cutting elements or tool inserts comprising PCD material are widely used in drill bits for boring into the earth in the oil and gas drilling industry. Rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes the thermal degradation of the diamond layer. The thermal degradation increases the wear rate of the cutter through increased cracking and spalling of the PCD layer as well as back conversion of the diamond to graphite causing increased abrasive wear.

Methods used to improve the abrasion resistance of a PCD composite often result in a decrease in impact resistance of the composite.

The most wear resistant grades of PCD usually suffer from a catastrophic fracture of the cutter before it has worn out. During the use of these cutters, cracks grow until they reach a critical length at which catastrophic failure occurs, namely, when a large portion of the PCD breaks away in a brittle manner. These long, fast growing cracks encountered during use of conventionally sintered PCD, result in short tool life.

Furthermore, despite their high strength, polycrystalline diamond (PCD) materials are usually susceptible to impact fracture due to their low fracture toughness. Improving fracture toughness without adversely affecting the material's high strength and abrasion resistance is a challenging task.

There is therefore a need for a super hard composite such as a PCD composite that has good or improved abrasion, fracture and impact resistance and a method of forming such composites.

SUMMARY

Viewed from a first aspect there is provided a method of forming a super hard polycrystalline construction, comprising:

-   -   forming a liquid suspension of a first mass of graphene and a         mass of particles or grains of super hard material;     -   dispersing the graphene and mass of super hard particles or         grains in the liquid suspension to form a substantially         homogeneous suspension;     -   drying the suspension to form an admix of the graphene and super         hard grains or particles;     -   forming a pre-sinter assembly comprising the admix;     -   treating the pre-sinter assembly in the presence of a         catalyst/solvent material for the super hard grains at an         ultra-high pressure of around 5 GPa or greater and a temperature         to sinter together the grains of super hard material to form a         body of polycrystalline super hard material comprising a first         fraction of super hard grains and a second fraction of diamond         grains, the super hard grains exhibiting inter-granular bonding         and defining a plurality of interstitial regions therebetween;     -   the graphene being at least partially converted to diamond         during the sintering stage to form the second fraction; wherein     -   the super hard grains in the first fraction are bonded along at         least a portion of the peripheral surface to at least a portion         of a plurality of diamond grains in the second fraction;     -   the super hard grains in the first fraction having a greater         average grain size than the average grain size of the grains in         the second fraction, the average grain size of the diamond         grains in the second fraction being between around 60 nm to         around 1 micron.

Viewed from a second aspect there is provided a super hard polycrystalline construction comprising:

-   -   a body of polycrystalline super hard material comprising a first         fraction of super hard grains and a second fraction of super         hard grains, the first fraction having a greater average grain         size than the super hard grains in the second fraction;     -   the super hard grains in the first and second fraction having a         peripheral surface; wherein     -   the super hard grains in the first fraction are bonded along at         least a portion of the peripheral surface to at least a portion         of a plurality of super hard grains in the second fraction;     -   the super hard grains in the second fraction being arranged to         space one or more adjacent grains in the first fraction by a         distance of between around 60 nm to around 1 micron.

BRIEF DESCRIPTION OF THE DRAWINGS

Versions will now be described by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of an example PCD cutter element or construction for a drill bit for boring into the earth;

FIG. 2 is a schematic cross-section of a portion of a conventional PCD micro-structure with interstices between the inter-bonded diamond grains filled with a non-diamond phase material;

FIG. 3 is a schematic cross-section of a portion of an example PCD micro-structure;

FIG. 4 is a plot showing the results of a vertical borer test comparing a conventional PCD cutter element and three example cutter elements; and

FIG. 5 is a plot showing the results of an analysis of diamond density comparing a conventional PCD cutter element and an example cutter element.

The same references refer to the same general features in all the drawings.

DESCRIPTION

As used herein, a “super hard material” is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of super hard materials.

As used herein, a “super hard construction” means a construction comprising a body of polycrystalline super hard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be free-standing and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline super hard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. As used herein, “interstices” or “interstitial regions” are regions between the diamond grains of PCD material. In some examples of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. In some examples of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

A “catalyst material” for a super hard material is capable of promoting the growth or sintering of the super hard material.

The term “substrate” as used herein means any substrate over which the super hard material layer is formed. For example, a “substrate” as used herein may be a transition layer formed over another substrate.

As used herein, the term “integrally formed” regions or parts are produced contiguous with each other and are not separated by a different kind of material.

An example of a super hard construction is shown in FIG. 1 and includes a cutting element 1 having a layer of super hard material 2 formed on a substrate 3. The substrate 3 may be formed of a hard material such as cemented tungsten carbide. The super hard material 2 may be, for example, polycrystalline diamond (PCD), or a thermally stable product such as thermally stable PCD (TSP). The cutting element 1 may be mounted into a bit body such as a drag bit body (not shown) and may be suitable, for example, for use as a cutter insert for a drill bit for boring into the earth.

The exposed top surface of the super hard material opposite the substrate forms the cutting face 4, also known as the working surface, which is the surface which, along with its edge 6, performs the cutting in use.

At one end of the substrate 3 is an interface surface 8 that forms an interface with the super hard material layer 2 which is attached thereto at this interface surface. As shown in the example of FIG. 1, the substrate 3 may be generally cylindrical and has a peripheral surface 14 and a peripheral top edge 16.

The super hard material may be, for example, polycrystalline diamond (PCD) and the super hard particles or grains may be of natural and/or synthetic origin.

The substrate 3 may be formed of a hard material such as a cemented carbide material and may be, for example, cemented tungsten carbide, cemented tantalum carbide, cemented titanium carbide, cemented molybdenum carbide or mixtures thereof. The binder metal for such carbides suitable for forming the substrate 3 may be, for example, nickel, cobalt, iron or an alloy containing one or more of these metals. Typically, this binder will be present in an amount of 10 to 20 mass %, but this may be as low as 6 mass % or less. Some of the binder metal may infiltrate the body of polycrystalline super hard material 2 during formation of the compact 1.

As shown in FIG. 2, during formation of a conventional polycrystalline composite construction, the diamond grains are directly interbonded to adjacent grains and the interstices 24 between the grains 22 of super hard material such as diamond grains in the case of PCD, may be at least partly filled with a non-super hard phase material. This non-super hard phase material, also known as a filler material, may comprise residual catalyst/binder material, for example cobalt, nickel or iron. The typical average grain size of the diamond grains 22 is larger than 1 micron and the grain boundaries between adjacent grains is therefore typically between micron-sized diamond grains, as shown in FIG. 2.

The working surface or “rake face” 4 of the polycrystalline composite construction 1 is the surface or surfaces over which the chips of material being cut flow when the cutter is used to cut material from a body, the rake face 4 directing the flow of newly formed chips. This face 4 is commonly also referred to as the top face or working surface of the cutting element as the working surface 4 is the surface which, along with its edge 6, is intended to perform the cutting of a body in use. It is understood that the term “cutting edge”, as used herein, refers to the actual cutting edge, defined functionally as above, at any particular stage or at more than one stage of the cutter wear progression up to failure of the cutter, including but not limited to the cutter in a substantially unworn or unused state.

As used herein, “chips” are the pieces of a body removed from the work surface of the body being cut by the polycrystalline composite construction 1 in use.

As used herein, a “wear scar” is a surface of a cutter formed in use by the removal of a volume of cutter material due to wear of the cutter. A flank face may comprise a wear scar. As a cutter wears in use, material may progressively be removed from proximate the cutting edge, thereby continually redefining the position and shape of the cutting edge, rake face and flank as the wear scar forms.

As shown in FIG. 3, in a polycrystalline composite construction according to a first example, the super hard material 2′ comprises a first fraction 30 of super hard grains or particles, a second fraction 32 of super hard grains or particles, and a third fraction 34 of super hard grains or particles. The first fraction 30 has a greater average grain size than that of the second and third fractions 32, 34. The grains of the first fraction 30 are bonded along a portion of their peripheral outer surface to plurality of grains of the third fraction 34, as are the grains of the second fraction 32 and may be spaced from adjacent grains in the first fraction by one or more grains in the third fraction 34.

In some examples, the average grain size of the third fraction 34 may be around 60 nm to around 1 micron.

Non-super hard phase material 24 such as residual catalyst/binder may remain in a number of the interstices between adjacent super hard grains 32, 34, 36, and the average binder pool size of these interstices may be smaller than in conventional PCD such as that shown in FIG. 2.

As used herein, a PCD grade is a PCD material characterised in terms of the volume content and size of diamond grains, the volume content of interstitial regions between the diamond grains and composition of material that may be present within the interstitial regions. A grade of PCD material may be made by a process including providing an aggregate mass of diamond grains having a size distribution suitable for the grade, optionally introducing catalyst material or additive material into the aggregate mass, and subjecting the aggregated mass in the presence of a source of catalyst material for diamond to a pressure and temperature at which diamond is more thermodynamically stable than graphite and at which the catalyst material is molten. Under these conditions, molten catalyst material may infiltrate from the source into the aggregated mass and is likely to promote direct intergrowth between the diamond grains in a process of sintering, to form a PCD structure. The aggregate mass may comprise loose diamond grains or diamond grains held together by a binder material and said diamond grains may be natural or synthesised diamond grains.

Different PCD grades may have different microstructures and different mechanical properties, such as elastic (or Young's) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K₁C toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

All of the PCD grades may comprise interstitial regions filled with material comprising cobalt metal, which is an example of catalyst material for diamond.

The PCD structure 2′ of examples may comprise two or more PCD grades.

The grains of super hard material may be, for example, diamond grains or particles. In the starting mixture prior to sintering they may be, for example, bimodal, that is, the feed comprises a mixture of a coarse fraction of diamond grains and a fine fraction of diamond grains having a smaller average grain size than the coarser fraction. By “average particle or grain size” it is meant that the individual particles/grains have a range of sizes with the mean particle/grain size representing the “average”. The average particle/grain size of the fine fraction is less than the size of the coarse fraction.

Some examples may comprise a wide bi-modal size distribution between the coarse and fine fractions of super hard material, and some examples may include three or even four or more size modes which may, for example, be separated in size by an order of magnitude.

Sizing of diamond particles/grains into fine fraction, coarse fraction, or other sizes in between, may be through known processes such as jet-milling of larger diamond grains and the like.

In some examples, the binder catalyst/solvent may comprise cobalt or some other iron group elements, such as iron or nickel, or an alloy thereof. Carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table are other examples of non-diamond material that might be added to the sinter mix. In some examples, the binder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in composition and, thus, may be include any of the Group IVB, VB, or VIB metals, which are pressed and sintered in the presence of a binder of cobalt, nickel or iron, or alloys thereof. In some embodiments, the metal carbide is tungsten carbide.

The cutter of FIG. 1 having the microstructure of FIG. 3 may be fabricated, for example, as follows.

Graphenes with various x-y dimensions and z dimensions are thoroughly dispersed in a liquid, such as deionized water or an organic solvent, for example ethanol, with or without a surfactant, using a sonication process by inserting an ultrasonic probe into the liquid. The dispersions may be pH adjusted to render the mixture slightly acidic which may assist in inhibiting agglomeration or aggregation of the graphenes. Diamond grits are added to the mixture and a sintering agent such as cobalt may also be added at this stage. The mixture is then subjected to a further sonication process by applying the ultrasonic probe into the mixture for a further period of time to form a homogeneous mixture. The resulting mixture is then dried using a fast drying process to maintain the homogeneity. An example of a suitable drying process to form graphene/diamond clusters, in which the graphenes are either coated on the surface of the diamond grits or the diamond grits are incorporated between graphene layers may be freeze drying using, for example, liquid nitrogen, or spray drying, or spray granulation. The clusters in the form of an admix powder are then sintered under conventional diamond sintering conditions, for example at a pressure of around 6.8 GPa, and temperature of around 1300 degrees C., in some examples with a preformed WC substrate, and the graphene is at least partially converted into diamond to form a PCD structure such as that shown in FIG. 3.

Whilst not wishing to be bound by a particular theory, it is believed that the thorough dispersion of the graphene and diamond grits in the initial dispersion mixture prior to drying may assist in achieving the desired microstructure in which the grains of the first fraction 30 are bonded along a portion of their peripheral outer surface to plurality of grains of a fraction 34 formed by the conversion during sintering of the graphenes into diamond having an average grain size of between around 60 nm to around 1 micron, the graphene. Various techniques may be used to achieve this dispersion and homogenous mixture, such as ultrasonic dispersing, ball milling, homogenization, and jet milling techniques.

Also, a fast drying process to dry the graphene/diamond suspension without agglomeration may assist in achieving the desired microstructure in the sintered product. Suitable drying techniques to assist in inhibiting agglomeration of the graphene materials during drying may include, for example, freeze drying, spray freeze drying, spray drying, and spray granulation or spray freeze granulation techniques.

In the example in which a spray drying technique is used, a suitable inlet temperature may be, for example, around 120 deg C., and an outlet temperature of around 50-56 deg C., and a feeding rate of around 5.8 ml/min may be used.

In the example in which a freeze drying technique is used, the admix may be in the form of a homogeneous paste which is frozen using liquid nitrogen, and is then placed into a freeze dryer for several days until the paste is thoroughly dried. The freeze drying conditions may be optimized for different solvents, for example, for deionized water, the preferred operation temperature is −55 deg C. plus/minus 5 deg C., and the preferred pressure is between around 50 to 500 microbar.

In some examples, other nanomaterials, such as nanodiamond may be introduced together with the graphene materials into the admix.

In some examples, a surfactant may be added to the suspension mixture such as a non-ionic or cationic surfactant. In some examples, a polymeric stabiliser having, for example, a viscosity close to the viscosity of water may be added to the suspension mixture.

In some examples, the admix material comprising the graphene and diamond admix, and the carbide material for forming the substrate plus any additional sintering aid/binder/catalyst are applied as powders and sintered simultaneously in a single UHP/HT process. The admix of graphene and diamond grains, and mass of carbide powder material are placed in an HP/HT reaction cell assembly and subjected to HP/HT processing. The HP/HT processing conditions selected are sufficient to effect intercrystalline bonding between the diamond grains and to at least partially convert the graphene into diamond grains as well as, optionally, the joining of sintered particles to the cemented metal carbide support. In one example, the processing conditions generally involve the imposition for about 3 to 120 minutes of a temperature of at least about 1200 degrees C. and an ultra-high pressure of greater than about 5 GPa.

In another example, the substrate may be pre-sintered in a separate process before being bonded to the super hard material in the HP/HT press during sintering of the ultrahard polycrystalline material.

In a further example, both the substrate and a body of polycrystalline super hard material are pre-formed. The preformed body of polycrystalline super hard material is then placed in the appropriate position on the upper surface of the preform carbide substrate (incorporating a binder catalyst), and the assembly is located in a suitably shaped canister. The assembly is then subjected to high temperature and pressure in a press, the order of temperature and pressure being again, at least around 1200 degrees C. and 5 GPa respectively. During this process the graphene is at least partially converted into diamond and solvent/catalyst may migrate from the substrate into the body of super hard material and acts as a binder-catalyst to effect intergrowth in the layer and also serves to bond the layer of polycrystalline super hard material to the substrate.

In some examples, the cemented carbide substrate may be formed of tungsten carbide particles bonded together by the binder material, the binder material comprising an alloy of Co, Ni and Cr. The tungsten carbide particles may form at least 70 weight percent and at most 95 weight percent of the substrate. The binder material may comprise between about 10 to 50 wt. % Ni, between about 0.1 to 10 wt. % Cr, and the remainder weight percent comprises Co.

The PCD construction 1 described with reference to FIGS. 1 and 3, may be further processed after sintering. For example, catalyst material may be removed from a region of the PCD structure adjacent the working surface or the side surface or both the working surface and the side surface. This may be achieved by treating the PCD structure with acid to leach out catalyst material from between the diamond grains, or by other methods such as electrochemical methods. A thermally stable region, which may be substantially porous, extending a depth of at least about 50 microns or at least about 100 microns from a surface of the PCD structure, may thus be formed which may further enhance the thermal stability of the PCD element.

Furthermore, the PCD body in the structure of FIG. 1 comprising a PCD structure bonded to a cemented carbide support body may be treated or finished by, for example, grinding, to provide a PCD element which is substantially cylindrical and having a substantially planar working surface, or a generally domed, pointed, rounded conical or frusto-conical working surface. The PCD element may be suitable for use in, for example, a rotary shear (or drag) bit for boring into the earth, for a percussion drill bit or for a pick for mining or asphalt degradation.

In addition, after sintering, the polycrystalline super hard constructions may be ground to size and may include, if desired, a chamfer, for example of around 45 degrees and of approximately 0.4 mm height measured parallel to the longitudinal axis of the construction.

Some versions are discussed in more detail below with reference to the following example, which is not intended to be limiting.

EXAMPLE

As used herein, graphene and exfoliated graphene materials will be referred to as ‘graphene materials’, and these are two dimensional soft materials which may at least partially convert into fine grain sized diamond materials under conventional diamond sintering pressures.

5 g of C500 graphene (Xscience) was dispersed in 50 ml of deionised water together with 1 g of a surfactant (such as polyvinyl alcohol) to form a suspension. The suspension was then treated with an ultrasonic probe for at least around 15 minutes until the graphene materials were thoroughly dispersed, and then 2 g of SP cobalt was added into the suspension with a further sonication process applied for around 5 minutes, using the ultrasonic probe to disperse the materials.

66.5 g of diamond grains having an average grain size of around 22 microns (Grade 22) and 28.5 g of diamond grains having an average grain size of around 2 microns (Grade 2) were then introduced into the suspension and the mixture was subjected to around 5 minutes further sonication treatment, as above.

The resulting suspension was then passed through an atomized nozzle (having a pressure of around 3 Bar), with a feed rate of around 6 ml/min, and sprayed into a container of liquid nitrogen to freeze the mixture immediately and form frozen granulates. After completion of the atomization, the frozen granulates were collected and placed into a freeze dryer for at least around 2 days to remove water content and form an admix powder. The graphene addition comprised around 5 wt % of the mixture. 2.1 g of the admix powder was then placed into a canister with a pre-formed cemented WC substrate to form a pre-sinter assembly, which was then loaded into a press and subjected to an ultra-high pressure and a temperature at which the super hard material is thermodynamically stable to sinter the super hard grains. The pressure to which the assembly was subjected was about 6.8 GPa and the temperature was at least about 1,200 degrees centigrade. The sintered PCD construction was then removed from the canister.

A conventional PCD cutter construction formed of the same mixture of diamond grain sizes (grades) as set out in the above example, but without adding the C500 type graphene was prepared in a conventional manner for forming PCD by mixing using conventional milling and mixing techniques and the mixture was sintered under the same conditions as above for the example containing the C500 type graphene, with a pre-formed cemented carbide substrate.

Two further example cutters were formed by the above described method used for the C500 type graphene but replacing the C500 type graphene with, in one example, C300 type graphene and, in another example, H15 type graphene.

The prepared PCD constructions formed according to the above methods were compared in a vertical boring mill test. The results are shown in FIG. 4.

The first PCD construction tested was that formed of the conventional diamond grain mixture (without any graphene additions) and the results are shown in FIG. 4 by line 100. The second PCD construction tested was a first example formed with the C300 type graphene described above and the results are shown in FIG. 4 by line 200. The third PCD construction tested was a second example formed of the C500 type graphene described above and the results are shown in FIG. 4 by line 300. The fourth PCD construction tested was a third example formed of the H15 type graphene described above and the results are shown in FIG. 4 by line 400. In this test, the wear flat area was measured as a function of the number of passes of the construction boring into the workpiece and the results obtained are illustrated graphically in FIG. 4.

The results provide an indication of the total wear scar area plotted against cutting length. It will be seen that the PCD constructions formed according to the second and third examples (lines 300 and 400) were able to achieve a significantly greater cutting length than that occurring in the conventional PCD compact (shown by line 100 in FIG. 4) which was subjected to the same test for comparison and all of the PCD constructions formed according to the examples (lines 200, 300 and 400) were able to achieve a significantly smaller wear scar area than that occurring in the conventional PCD compact (shown by line 100 in FIG. 4).

Thus it will be seen from FIG. 4 that the PCD construction formed with the inclusion of a high BET surface area graphene (C500 denoted by line 300) in the admix prior to sintering, that is, graphene having a BET surface area of above 500 m²/g, showed a significant improvement in both cutting distance and abrasion resistance over both the conventional PCD construction (line 100) and the PCD constructions formed with the inclusion of a medium BET surface area graphene (100 to 500 m²/g) and a low BET surface area graphene (less than 100 m²/g) in the admix prior to sintering.

An additional sample according to the above example containing 5 wt % of the C500 type graphene was produced as was an additional reference cutter which was formed of the same mixture of diamond grain sizes (grades) as set out in the above example, but without adding the graphene, and it was prepared in a conventional manner for forming PCD by mixing using conventional milling and mixing techniques and the mixture was sintered with a pre-formed cemented carbide substrate under the same conditions as above for the example containing the graphene addition. Polished sections of the constructions were then analysed using conventional SEM techniques to determine the respective diamond densities in the constructions. The results are shown in FIG. 5.

It will be seen from FIG. 5 that the sample with the graphene addition improved diamond density by around 1%.

Whilst not wishing to be bound by a particular theory, it is believed that the benefits of adding graphene to improve the abrasion resistance and working life of the PCD construction in use may be influenced by both the surface area of the graphene used in the starting material and the diamond grain size. In some examples, the BET surface area of the graphene may be larger than around 50 m²/g, for example larger than around 100 m²/g, or larger than around 300 m²/g, and even around 500 m²/g or more. In addition, in some examples, the average grain size of at least one of the diamond fractions in the admix may be around 6 microns or less, or around 4 microns or less, or around 2 microns or less.

Thus, examples of a PCD material may be formed having that a combination of high abrasion and fracture performance which is surprising considering the reduction in diamond contiguity in those PCD constructions which is shown schematically in FIG. 3.

Diamond contiguity is an important performance indicator, as it indicates the degree of intergrowth or bonding between the diamond particles, and all else being equal the higher the diamond contiguity the better the cutter performance. Higher diamond contiguity is normally associated with high diamond content which in turn results in lower binder content, as the high diamond content translates into low porosity and therefore low binder content, as the binder occupies the pores.

According to classic materials science of composite materials, low binder content results in low fracture toughness, as it is normally the hard grains (in this case diamond) that imparts hardness to the composite material, and the more ductile binder (in PCD, normally Co-WC) that imparts toughness to the composite material.

Therefore, high diamond content and low binder content are expected to be associated with increased hardness and decreased toughness, so that failure due to fracture or spalling of the PCD is expected to increase.

It was therefore surprising to find that PCD with improved wear performance may be obtained by adding graphene in the starting mixture which at least in part converts to form nanodiamond sized diamond grains during sintering at HPHT bridging the surfaces of at least some of the adjacent larger diamond grains, as is evidenced by the results of an analysis of the wear performance of the PCD material shown in FIG. 4.

It is further believed that these nanodiamond bridges between the larger diamond grains may assist in arresting cracks which may attempt to propagate through the material in use.

The microstructure of the PCD constructions formed according to one or more of the above described example methods may be determined using conventional image analysis techniques such as scanning electron micrographs (SEM) taken using a backscattered electron signal.

The homogeneity or uniformity of the PCD structure may be quantified by conducting a statistical evaluation using a large number of micrographs of polished sections, as may the diamond density, the latter of which is shown in FIG. 5 for a sample with a 5 wt % addition of C500 graphene in the starting mixture. The distribution of the filler phase, which is easily distinguishable from that of the diamond phase using electron microscopy, can then be measured in a method similar to that disclosed in EP 0 974 566 (see also WO2007/110770). This method allows a statistical evaluation of the average thicknesses of the binder phase along several arbitrarily drawn lines through the microstructure. This binder thickness measurement is also referred to as the “mean free path” by those skilled in the art.

While various versions have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular examples or versions disclosed.

For example, in some embodiments of the method, the PCD material may be sintered for a period in the range from about 1 minute to about 30 minutes, in the range from about 2 minutes to about 15 minutes, or in the range from about 2 minutes to about 10 minutes.

In some examples of the method, the sintering temperature may be in the range from about 1,200 degrees centigrade to about 2,300 degrees centigrade, in the range from about 1,400 degrees centigrade to about 2,000 degrees centigrade, in the range from about 1,450 degrees centigrade to about 1,700 degrees centigrade, or in the range from about 1,450 degrees centigrade to about 1,650 degrees centigrade.

In one example, the method may include removing residual metallic catalyst/binder material for diamond from interstices between the diamond grains of the PCD material. In some examples, the PCD structure may have a region adjacent a surface comprising at most about 2 volume percent of catalyst material for diamond.

In further examples, the PCD structure may additionally have a region remote from the surface comprising greater than about 2 volume percent of catalyst material for diamond. In some such examples, the region adjacent the surface may extend to a depth of at least about 20 microns, at least about 80 microns, at least about 100 microns or even at least about 400 microns from the surface, or greater. 

What is claimed is:
 1. A method of forming a super hard polycrystalline construction, comprising: forming a liquid suspension of a first mass of graphene and a mass of particles or grains of super hard material; dispersing the graphene and mass of super hard particles or grains in the liquid suspension to form a substantially homogeneous suspension; drying the suspension to form an admix of the graphene and super hard grains or particles; forming a pre-sinter assembly comprising the admix; treating the pre-sinter assembly in the presence of a catalyst/solvent material for the super hard grains at an ultra-high pressure of around 5 GPa or greater and a temperature to sinter together the grains of super hard material to form a body of polycrystalline super hard material comprising a first fraction of super hard grains and a second fraction of diamond grains, the super hard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween; the graphene being at least partially converted to diamond during the sintering stage to form the second fraction; wherein the super hard grains in the first fraction are bonded along at least a portion of the peripheral surface to at least a portion of a plurality of diamond grains in the second fraction; the super hard grains in the first fraction having a greater average grain size than the average grain size of the grains in the second fraction, the average grain size of the diamond grains in the second fraction being between around 60 nm to around 1 micron.
 2. The method of claim 1, wherein the step of providing a mass of super hard material comprises providing a mass of natural and/or synthetic diamond grains, the super hard polycrystalline construction forming a polycrystalline diamond (PCD) construction.
 3. (canceled)
 4. The method of claim 1, further comprising treating the super hard construction to remove at least a portion of residual binder/catalyst from at least a portion of interstitial spaces between interbonded super hard grains.
 5. The method of claim 1, wherein the step of forming a liquid suspension of a first mass of graphene and a mass of particles or grains of super hard material comprises dispersing the graphene and superhard particles or grains in deionized water.
 6. The method of claim 1, wherein the step of forming a liquid suspension of a first mass of graphene and a mass of particles or grains of super hard material comprises dispersing the graphene and superhard particles or grains in an organic solvent.
 7. The method of claim 6, wherein the organic solvent comprises ethanol.
 8. The method of claim 1, wherein the step of forming a liquid suspension further comprises adding a surfactant to the liquid suspension.
 9. The method of claim 8, wherein the surfactant comprises a non-ionic or cationic surfactant.
 10. The method of claim 1, wherein the step of dispersing the graphene and mass of super hard particles or grains in the liquid suspension to form a substantially homogeneous suspension comprises applying to the liquid suspension one or more of: a sonication process; an ultrasonic dispersion process; a homogenization process; and/or a jet milling process.
 11. The method of claim 1, wherein the step of dispersing the graphene and mass of super hard particles or grains in the liquid suspension to form a substantially homogeneous suspension comprises adjusting the pH of the liquid suspension to render the suspension acidic to assist in inhibiting agglomeration or aggregation of the graphene.
 12. The method of claim 1, wherein the step of drying the suspension to form an admix of the graphene and super hard grains or particles comprises one or more of drying the suspension using freeze drying spray freeze drying, spray drying, spray granulation, and/or spray freeze granulation.
 13. The method of claim 1, wherein the graphene has a BET surface area of around 50 m²/g or more, or around 100 m²/g or more, or around 300 m²/g or more, or around 500 m²/g or more.
 14. The method of claim 13, wherein the average grain size of the super hard grains or particles in the admix is around 6 microns or less, or around 4 microns or less, or around 2 microns or less.
 15. A super hard polycrystalline construction comprising: a body of polycrystalline super hard material comprising a first fraction of super hard grains and a second fraction of super hard grains, the first fraction having a greater average grain size than the super hard grains in the second fraction; the super hard grains in the first and second fraction having a peripheral surface; wherein the super hard grains in the first fraction are bonded along at least a portion of the peripheral surface to at least a portion of a plurality of super hard grains in the second fraction; the super hard grains in the second fraction being arranged to space one or more adjacent grains in the first fraction by a distance of between around 60 nm to around 1 micron.
 16. (canceled)
 17. The super hard polycrystalline construction of claim 15, wherein the body of super hard material comprises inter-bonded super hard grains comprising natural and/or synthetic diamond grains, the super hard polycrystalline construction forming a polycrystalline diamond (PCD) construction.
 18. The super hard polycrystalline construction of claim 15, wherein the PCD construction further comprises a non-super hard phase comprising a binder phase located in interstitial spaces between the inter-bonded diamond grains.
 19. The super hard polycrystalline construction according to claim 18, wherein the binder phase comprises cobalt, and/or one or more other iron group elements, such as iron or nickel, or an alloy thereof, and/or one or more carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table. 20.-22. (canceled)
 23. The super hard polycrystalline construction according to claim 15, wherein at least a portion of the body of super hard material is substantially free of a catalyst material for diamond, said portion forming a thermally stable region.
 24. The super hard polycrystalline construction as claimed in claim 23, wherein the thermally stable region comprises at most 2 weight percent of catalyst material for diamond.
 25. The super hard polycrystalline construction of claim 15, wherein the first fraction comprises a mass of super hard abrasive grains having two or more different average grain sizes. 26.-33. (canceled) 